A print head (FIGS. 1A, 1B, 1C and 1D) of a dot printer comprises a plurality of needles 1 (1-1 to 1-9 as illustrated), and uses a drive unit such as a solenoid to project necessary needles 1-n selectively for printing, thereby forming dots on a paper 2 to print it.
A plurality of needles 1 have their respective proximal ends on the drive side arranged on a circumference C and are converged so that their tip ends for forming dots are arranged straight in a dotting position p, thus forming a cone-shaped needle bunch 3 (FIG. 1A). This needle bunch is kept in the cone-shaped configuration by an intermediate needle guide 4 located halfway in its longitudinal direction and a tip needle guide 5 at its tip end portion. In some cases, there may exist a plurality of intermediate needle guides 4.
Since a clearance d is defined between the tip needle guide 5 and a surface of the paper 2 (FIG. 1B), needle holes in the tip needle guide 5 are not arranged straight in this region but are apparently arranged in zigzag (FIG. 1D). Further, an angle between each needle 1 that constitutes the cone-shaped needle bunch 3 and a horizontal plane HP (plane perpendicular to the surface of the paper 2) will be referred to as a convergence angle (θn), and a direction from the proximal end of the needle 1 toward the tip end, as viewed from the front (viewed from the bottom of the cone of the needle guide toward its vertex), as a convergence direction (FIG. 2A). The intermediate needle guide and the tip needle guide will be referred to simply as a needle guide, in general.
The structure for guiding the needles 1 in the print head of the dot printer is essential to the maintenance of the accuracy of printing. The construction and material of the needle guide itself and a structure for its attachment to a head body have been repeatedly improved in various manners.
In a print head described in Japanese Utility Model Application Registered No. 2582809, for example, a plurality of intermediate needle guides are constructed separately, and the intermediate needle guides are arranged for movement with respect to a nose (head body) in the axial direction of the needles. Thus, the needles are prevented from being partially distributed when all the needles are set. In a tip wire bearing structure described in Japanese Patent Application Examined No. 61-8794, moreover, a hollow bearing structure is constructed having a front wall and a rear wall, and it is penetrated by the respective tip end portions of print wires in a direction perpendicular to the opposite walls.
For miniaturization and reduction in weight, conventional print heads are designed so that the guide wall thickness of the needle guide is thinned, and are formed of synthetic resin or other materials that are low-priced and light in weight. However, a print head of a dot printer requires durability such that the needles 1 can reciprocate about one hundred million times, and the needle guide naturally requires equal durability. On the other hand, the synthetic resin easily wears, and the guide wall thickness should be increased in some measure. Besides, it costs high to form through holes for the needles 1 in a direction such that the needles 1 are inclined as they penetrate the holes.
More specifically, the needles 1 of the needle bunch 3 have their respective different convergence angles (θn), and it is very hard to form slanting holes adapted to their respective convergence angles, in guide walls. Even if those holes can be formed, it is difficult to locate the needle guide in an accurate position in a body case when the needles 1 are passed and incorporated into the head body. Some of the needles 1 suffer “rubbing” between a plurality of needle guides, and a load on the needles becomes too heavy for high-accuracy printing.
Normally, therefore, the through holes for the needles 1 are simply formed in a direction perpendicular to the obverse and reverse surfaces of the guide walls, while the intermediate needle guides are mounted for movement in the longitudinal direction toward the head body. In this case, the needles 1 have the convergence angles (θn) for the through holes, so that the through holes should be lengthened in the convergence direction of the needles, depending on the guide wall thickness. Thereupon, if the guide wall thickness is increased to improve the durability of the needle guide, the length of the through holes in the convergence direction increases correspondingly, so that the operation of the needles 1 becomes unstable. Further, the durability fails to be improved despite the increase of the wall thickness.
This tendency is further enhanced if the inclination (convergence angle) of the needles is increased to reduce the longitudinal length of the print head. Thus, since the convergence angle of the needles is widened, the length of the needle holes in the convergence direction further increases, so that the through holes are enlarged (FIG. 2B).
The bearing structure described in Japanese Patent Application Examined No. 61-8794 is the hollow structure having the front wall and the rear wall. However, the print wires are passed through both the walls at right angles to them, and the wires have no angles equivalent to the convergence angles (θn) in the position of the bearing structure. Although this bearing structure has the first wall and the rear wall separate from each other, the same effect can be obtained when the longitudinal thickness is increased.
The tip needle guide suffers another problem that is aroused when the convergence angle (θn) varies. More specifically, the through holes of the needles 1 are inevitably enlarged, so that a plurality of through holes are located too close to each other to secure necessary spaces for molding, and some of the through holes communicate with each other (FIG. 2C). If this is done, the strength of the needle guide itself lowers. Further, projections at the junctions easily wear, and the through holes inevitably communicate with each other, so that guiding the tip end portions of the needles tends to be unstable. Thus, high-accuracy printing cannot be enjoyed with ease.